1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on recording materials such as sheet materials.
2. Description of the Related Art
A conventional image forming apparatus will be described with reference to FIG. 8, which is a sectional view of a conventional color image forming apparatus.
Referring to the drawing, a photosensitive drum 1 serving as an image bearing member is driven in the direction indicated by the arrow by a driving means (not shown); it is charged uniformly by a primary charger 2.
Then, a laser beam L which is in conformity with an yellow image is applied to the photosensitive drum 1 from an exposure device 3, whereby a latent image is formed on the photosensitive drum 1.
As the photosensitive drum 1 further rotates in the direction of the arrow, a developing device 4a, containing yellow toner, of four developing devices 4a (yellow), 4b (magenta), 4c (cyan), and 4d (black) supported by a rotation supporting means 11 rotates to come to be opposed to the photosensitive drum 1, and the image is visualized by the yellow developing device 4a selected.
An intermediate transfer belt 5 rotates in the direction of the arrow substantially at the same speed as the photosensitive drum. The toner image formed and borne on the photosensitive drum 1 undergoes primary transfer to the outer surface of the intermediate transfer belt 5 by a primary transfer bias applied to a primary transfer roller 8a. 
The above-described process is performed for the four colors: yellow (hereinafter referred to as Y), magenta (hereinafter referred to as M), cyan (hereinafter referred to as C), and black (hereinafter referred to as K), whereby a toner image of a plurality of colors is formed on the intermediate transfer belt 5.
Next, a transfer material is fed with a predetermined timing from a transfer material cassette 12 by means of pick-up rollers 13.
At the same time, a secondary transfer bias is applied to a secondary transfer roller 8b, and the toner image is transferred from the intermediate transfer belt 5 to the transfer material.
Further, the transfer material is conveyed by a conveyance belt 14 to a fixing device 6, where fusion and fixing are effected, whereby a color image is obtained.
The toner remaining on the intermediate transfer belt 5 is removed by an intermediate transfer belt cleaner 15.
On the other hand, the toner remaining on the photosensitive drum 1 is removed by a cleaning device 7 consisting of a well-known blade means.
When using the image forming apparatus described above, maintenance operations, such as toner replenishment, the disposal of waste toner, and the replacement of the photosensitive drum 1 when it has been worn.
In this example, the photosensitive drum 1, the primary charger 2, and the cleaning device 7 are integrated into a process cartridge A, and the developing devices 4a, 4b, 4c, and 4d are also in the form of a developing cartridge which is easily detachable with respect to the apparatus main body, so that the maintenance operations can be easily conducted by the user.
Generally speaking, in an electrophotographic image forming apparatus, fluctuations in the density characteristics of the printed image are caused by the fluctuations in characteristics due to the use environment, the developing device, the number of sheets on which printing has been effected by the photosensitive drum, the variation in sensitivity generated at the time of the production of the photosensitive drum, the variation in frictional charging characteristics generated at the time of the manufacturing of the toner, etc.
Although strenuous efforts have been put into stabilizing the characteristics in the variations and fluctuations, no satisfactory result has been achieved yet.
In particular, in a color image forming apparatus, it is necessary to adjust the conditions for the image formation in the four colors of Y, M, C, and K before the user can achieve a desired density and color balance.
In view of this, in the color image forming apparatus of this example, a plurality of toner images for detection are formed on the photosensitive drum 1 by varying the image forming condition stepwise, and the reflection light quantity thereof is measured by a density sensor 9. On the basis of the result of the measurement, an image forming condition which is likely to provide a desired density (reflection light quantity) is computed by a CPU 17 of the main body for image density control.
Thus, the CPU 17 and the density sensor 9 correspond to image formation condition computing means constituting elements of the present invention used in the embodiment described below.
Next, the density sensor 9 will be described with reference to FIG. 9, which is a schematic view of the density sensor applied to the image forming apparatus shown in FIG. 8.
The density sensor 9 is composed of a light emitting element 91 such as LED, a photoreceptor 92 such as a photo diode, and a holder 93. Infrared radiation from the light emitting element 91 is applied to a patch P on the photosensitive drum, and the reflected light therefrom is measured by the photoreceptor 92, whereby the density of the patch P is measured.
The reflected light from the patch P contains a regular reflection component and an irregular reflection component. The light quantity of the regular reflection component undergoes great fluctuations depending on the condition of the photosensitive drum surface underneath the patch and fluctuation in the distance between the sensor and the patch. Thus, when the reflected light from the patch to be measured contains a regular reflection component, the detection accuracy deteriorates to a marked degree.
In view of this, in the density sensor 9, in order that no regular reflection component from the patch P may impinge on the photoreceptor 92, the angle at which light is applied to the patch P is set to 45xc2x0 and the reception angle of the reflected light from the patch P is set to 0xc2x0 with respect to the normal I, thus measuring only the irregular reflection component.
Next, the image density control in the color image forming apparatus of this example will be described in detail.
First, the photosensitive drum 1 is charged by the primary charger 2 such that its surface potential becomes xe2x88x92600V.
The sensitivity of the photosensitive drum and the exposure amount of the laser are adjusted beforehand such that the potential of the laser exposure portion at normal temperature and normal humidity (23xc2x0 C., 60% Rh) is approximately xe2x88x92200V.
The developing bias is obtained by superimposing a rectangular wave (with a frequency of 2000 Hz, 1800 Vpp) on a DC voltage, as shown in FIG. 10. By making the DC voltage component Vdc variable, the toner development amount is controlled. FIG. 10 is a graph depicting the developing bias applied to the image forming apparatus shown in FIG. 8.
Further, prior to normal image formation, a plurality of toner image patches of 30 mm square are printed at intervals, as shown in FIG. 11, on the portion of the drum corresponding to the density sensor 9. FIG. 11 is a schematic diagram showing patches for density detection applied to the image forming apparatus shown in FIG. 8.
The image patches are each developed by developing biases with different DC voltage components, and reflection light quantity measurement is performed on each of them by the density sensor 9. In this example, the number of image patches is five, the DC component Vdc of the developing bias being varied from xe2x88x92300V to xe2x88x92500V in steps of 50V.
FIG. 12 shows an example of the result of reflection density measurement. FIG. 12 is a graph showing the relationship between reflection density and developing bias in the image forming apparatus shown in FIG. 8.
In this example, the target value of the reflection density of the toner (proper density value) is 1.4, and control is effected such that image formation is conducted under a developing condition estimated to be closest thereto (in this example, the DC voltage component of the developing bias).
In this example, reflection density data as indicated by the five points in FIG. 12 was obtained. The developing condition providing the reflection density of 1.4 lies in the section where the DC component Vdc is between xe2x88x92400V and xe2x88x92450V. Assuming that DC component is approximately proportional to reflection density in this section, it is to be assumed, through Interior division of the section between xe2x88x92400V and xe2x88x92450V, that the reflection density is 1.4 when the DC component is approximately xe2x88x92420V.
Thus, in this example, the DC component Vdc of the developing bias as an image formation condition is controlled to be xe2x88x92420V.
The above-described control is executed for each of the colors, Y, M, C, and K, whereby the image density control is completed.
The image density control is executed prior to image formation (printing) each time printing is performed on a predetermined number of sheets, when the power source of the main body is ON, when replacing the process cartridge A or the development cartridges (developing devices) 4a, 4b, 4c, and 4d, and when a printing command is received when the apparatus has not been in use for a long period of time.
While in this example the number of image patches is five, it is also possible to increase the number to vary the developing bias in more steps, thereby performing control more accurately.
When the variation in image density is too great to be coped with solely by adjusting the developing bias, it is also possible to perform control by combining other image formation conditions, such as charging condition and exposure condition (exposure amount).
The conventional color image forming apparatus, however, has the following problems.
As described above, in an electrophotographic image forming apparatus, the developing characteristics of the developing device and the photosensitive characteristics of the photosensitive drum fluctuate according to the condition of use of the apparatus, with the result that the image density varies.
In particular, when printing is conducted successively, the above-mentioned fluctuations in characteristics are more conspicuous, and the image density is greatly varied.
Thus, each time printing is performed on a fixed number of sheets, the above-described image density control is executed, whereby the image density is prevented from being too much deviated from the proper value.
However, even if such control is performed, fluctuation in density naturally occurs between image density control operations.
And, when the fluctuation in density occurs to a large degree, a marked difference in density is generated before and after the execution of the image density control.
This will be explained in detail with reference to FIG. 13, which is a graph showing the variation in image density when printing is successively executed in a conventional image forming apparatus.
In the drawing, the vertical axis indicates density, and the horizontal axis indicates the number of sheets on which printing is performed. Broken line A indicates the proper image density for the apparatus, and broken line B indicates how the image density will change when image density control is not conducted each time printing has been conducted on a fixed number of sheets.
At the left-hand end (x0) of the graph, image density control is effected, and the image density is adjusted to the proper density.
As can be seen from the graph, if density control is not executed each time printing has been performed on a fixed number of sheets, the image density will continue to increase to be greatly deviated from the proper density.
Thus, it is necessary to conduct image density control each time printing has been performed on a fixed number of sheets. Solid line C indicates how the image density changes when image density control is performed.
In this example, image density control is effected each time printing has been performed on 100 sheets. Density control is effected at points in time indicated by numerals X1 and X2 in the drawing.
By thus performing image density control, the image density is prevented from being greatly deviated from the proper density for a long period of time.
However, there occurs a marked fluctuation in density between the execution of image density control (indicated by X1 and X2 in the drawing).
Suppose the user successively conducts the printing of the same image before and after density control. For example, if printing is successively performed on twenty sheets, for example, from the 90th to 110th sheet, there is the possibility of the image density of the first ten sheets being greatly different from that of the ten sheets after density control.
In particular, in the case of a color image forming apparatus like that of this example, in which a full color image is reproduced by superimposing four color toner images one upon the other, a great variation in the density of a particular color (one of Y, M, C, and K) results in a marked change in the hue of the image which is very conspicuous.
FIG. 14 shows an example in which, conversely to the case of FIG. 13, image density is gradually reduced. As can be seen from this graph, a similar problem is involved also in this case. FIG. 14 is a graph showing the variation in image density when printing is successively executed in a conventional image forming apparatus.
The above problem might be coped with by frequently performing image density control. In that case, however, the requisite time for density control would cause a reduction in the printing speed. Moreover, that would involve the consumption of a lot of toner for density control.
It is an object of the present invention to provide an image forming apparatus which is capable of preventing an abrupt fluctuation in image density.
Another object of the present invention is to provide an image forming apparatus in which the difference in image density between an initial stage of use and a stage after long use is reduced.
Still another object of the present invention is to provide an image forming apparatus in which a reduction in image forming speed when changing an image formation condition is mitigated.
A further object of the present invention is to provide an image forming apparatus in which toner consumption for changing an image formation condition is restrained.
A further object of the present invention is to provide an image forming apparatus in which it is possible to change an image formation condition gradually and stepwise.
These and still other objects, advantages and benefits may be achieved using an image forming apparatus comprising:
an image forming means for forming an image on a recording material;
a detecting means for detecting an image density; and
a changing means for changing a former image formation condition for the image forming means to a next image formation condition for the image forming means on the basis of a detection result of the detecting means, wherein the changing means is capable of changing the former image formation condition so as to bring it close to the next image formation condition through a stepwise change of image formation condition.
Further objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.